1. Field of the Invention
The present invention relates to a method of visualising the orientation of therapeutic sound waves onto an area to be treated or processed, as well as to a device for carrying through this method.
2. Prior Art
Devices for the application of therapeutic sound waves are generally common:
For lithotripsy, for instance, pulsed pressure or shock waves generated inside or outside the body are employed whereas continuous sound waves are used, for instance, to heat tissue.
It is necessary in any case to direct the therapeutically efficient fraction of the sound fieldxe2x80x94which will be briefly referred to as xe2x80x9csound wave focusxe2x80x9d in the followingxe2x80x94onto the region requiring therapeutic treatment or processing, respectively. This may be done by moving the sound source and/or the patient as well as by influencing the distribution of pressure in space within the sound field and hence by displacement of the focus relative to the sound source.
This orientating operation, which will be referred to as xe2x80x9cpositioningxe2x80x9d in the following, should, as a rule, not be performed automatically but by the person applying the device, i.e. a physician, for instance, for a number of reasons such as reasons of liability. To enable the operator to perform such positioning a numerical or graphic indication is required that permits the operator can direct the sound wave focus onto the treatment site.
In lithotripter devices operating on pressure waves generated outside the body the indication is available, for instance, by mixing a crosshair cursor into a two-dimensional X-ray or ultrasonic image.
One solution that is simple from an engineering point of view but possibly inexpedient for the apparatus concept from an ergonomic point of view and with respect to the expenditure incurred by the structure consists in a mechanical connection of the imaging componentsxe2x80x94i.e. the X-ray tube/image amplifier or the ultrasonic transmitterxe2x80x94to the sound source so as to establish an invariable positional relationship between the imaging components and the ultrasound source.
With more recent developments it has been proposed to arrange the sound source in a mobile form and to detect the spatial positioning of the image-producing or imaging components relative to the sound source by suitable measuring techniques. An example of such a device is described in the German Patent DE-A-195 12 956 to which explicit reference is made with respect to all details not explained here.
As a fixed three-dimensional correlation between the imaging system and the sound source does no longer exist in that device there is no possibility to mix in fixed markers or the like. The position of the sound wave focus, which is now variable, must rather now be displayed or represented relative to the treatment site.
A simple solution in engineering terms consists in a numerical display of this information as spacing in a three-dimensional co-ordinate system. With such a concept, however, positioning becomes a time-consuming operation because the operator is bound to convert this numerical information into a three-dimensional movement.
Another positioning display is described in FIG. 4 of the German Patent DE-A-195 12 959. There the position of the sound wave focus is represented in a plane and on an axis orthogonal on this plane. There is the disadvantage, however, that the operator must permanently keep an eye on two movable markers in order to judge the position in space. Even though misinterpretation is avoided with this displayxe2x80x94which is not pseudo three-dimensionalxe2x80x94the conversion of the video information does not permit a rapid non-fatiguing positioning.
Moreover, for instance in computer games but not in medical applicationsxe2x80x94it is the state of the art to generate stereoscopic or pseudo three-dimensional representations by means of two small screens mounted in a helmet: the two screens supply the video information to the left and the right eye separately (xe2x80x9chead-mounted displaysxe2x80x9d).
Other known systems use only a single screen on which the video information for the left and the right eye is separated by various techniques (xe2x80x9cshutter glassesxe2x80x9d, polarisation glasses, red/green glasses).
It is known from these applications outside the bioengineering field that all people are not able to gather a three-dimensional impression from stereo displays. But even when a spatial impression can be developed fatiguing occurs after a major period of application, which may result in consequences such as misinterpretation.
For the indication of position in a medical application, however, non-fatiguing operation and reliable protection from misinterpretation must be demanded. After all, the success and the low level of side effects of a therapeutic operation are dependent on the correct conversion of the video information.
The present invention is based on the problem of providing a method of visualising the orientation of therapeutic sound waves onto a region to be treated or processed, respectively, as well as a device for carrying through this method, wherein the positioning information is provided by a graphic display easy to interpret.
One inventive method solving this problem is defined in claim 1. Improvements are the subject matters of claims 2 et seq. A device for carrying through the inventive method is defined in claim 23.
The invention starts out from the following finding:
The inventive approach is based on findings gathered in the design of three-dimensional bodies on computers (xe2x80x9ccomputer-aided designxe2x80x9d, xe2x80x9cCADxe2x80x9d) and on the application of virtual reality (xe2x80x9cVRxe2x80x9d) based thereon. The basic principle of VR techniques of representation resides in the fact that the perspective of the displayed bodies varies as the viewer""s position changes. In distinction from the stereoscopic display the static image alone does not yet create a three-dimensional impression because the left and the right eye receive each the same information.
When, by contrast, a succession of images is generated where the viewer""s site is continuously varied the impression is created as if the viewer moves through an arrangement of objects (hereinafter referred to as xe2x80x9cscenarioxe2x80x9d).
The human brain is able to conclude the size and the relative position of the displayed objects from such a succession of images. The situation is equal when the viewer is permitted to grasp objects in the scenario apparently (virtual hand) and to move them relative to each other. The similarity of the real scenario with the scenario generated on the screen creates the illusion on the viewer""s part as if he were actually in the scenario (xe2x80x9cimmersionxe2x80x9d effect).
It is the basic idea underlying the present invention to utilise such display techniques for positioning the sound wave focus relative to a treatment site.
To this end the sound wave source and the spatial region through which the sound wave are propagating as well as the region to be treated or processed (hereinafter referred to as treatment site) are displayed in a perspective view by assignment of virtual three-dimensional figures. When the site or the orientation of the sound wave source and/or the treatment site and/or the site or the orientation of the screen is varied the perspective view varies equally with the correct movement.
The representation reflects the real position of the sound source and the treatment site in a perspectively correct view so as to provide the operator with a correct impression of the size and the spacings of the figures. For orientation onto the treatment site the figure corresponding to the sound wave focus is moved in the represented scenario just like in reality. The operator therefore gets the impression as if he were in the virtual scenario, guiding the sound source there towards the treatment site (immersion, virtual hand).
It is particularly preferable that the perspective view is changed or can be manually varied when the operator""s site varies, in correspondence with the varied view of the scenario for the operator. In particular, the signals representing the position of the sound source in space, which are obtained from a measuring system detecting the relative position of the sound source and the treatment site or the locating means, respectively, are converted into an equivalent virtual movement almost in real time.
In the case of focussed therapeutic sound waves the waves are preferably represented as a cone whose envelope corresponds approximately to the transition between the focussed wave and the marginal diffracted wave. In an improvement, the envelope of the displayed cone is displayed with a surface so open that the treatment site will not be covered. In particular, the envelope of the displayed cone can be represented in a semi-transparent form.
It is furthermore preferred that the treatment site is displayed approximately with the size in which the sound waves produce a therapeutic or processing effect. With focussed sound waves, in particular, the treatment site can be displayed as sphere or ellipsoid.
A particularly preferred embodiment of the inventive method comprises an approach indicator function that represents the approach to or congruence with the region where the sound waves produce a therapeutic or processing effect. Using an additional numerical display or other appropriate means, the operator obtains quantitative information about the precision with which positioning has been performed.
The approach indicator function may be implemented with acoustic meansxe2x80x94e.g. by a variation of the pitch (sound frequency) and/or the sound repetition rate. Moreover, the approach indicator function can be implemented by a change of the color in which at least one of the virtual three-dimensional figures is displayed. It is moreover possible to provide the approach indicator function in a numerical form.
To avoid fatiguing it is preferred that the displayed objects present surfaces (xe2x80x9crenderingxe2x80x9d). In such a case it is expedient to simulate the incidence and reflections of light on the displayed surfaces, for instance in the form in which these effects would occur in the case of an endoscopic or surgical operation.
To this end a virtual source of illumination may be provided on the ceiling of the treatment or processing room. As an alternative or additionally a virtual source of illumination may be arranged in the center of the treatment site. One light source is located above the operator. This corresponds to the light incidence that the operator is accustomed to so that sections illuminated in a brighter color on the cone or sphere surfaces will be estimated as being on top in the correct position. A second light source is located in the middle of the treatment site so that brightening occurs in the area of the tip of the cone when the target is approached. With such a design undesirable strong shading is avoided in an approach to the target from the top.
To facilitate the operator""s orientation it is expedient to mix stationary parts of the treatment or processing room into the displayed image. The stationary elements may be a treatment rest and/or a positioning means for a locating meansxe2x80x94e.g. an X-ray C arc or an ultrasonic locating means.
It is furthermore expedient for the user that a zoom function is implemented which may be used to display the environment of the treatment site in an enlarged view. Moreover, the position of the treatment site may be displayed as a function of the output signal of a locating means.